[RS4GC] Simplify handling of Constants in findBaseDefiningValue(). NFC.

Summary:
Previously there were three conditionals, checking for global
variables, undef values and everything constant except these two, all three
returning the same value.  This commit replaces them by one conditional.

Reviewers: reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15818

llvm-svn: 256812